Inverters have been installed in many types of electric equipment such as a motor used in a vehicle, as an efficient variable-speed control unit. However, the inverters are switched at a frequency of several kHz to tens of kHz, to cause a surge voltage at every pulse thereof. Inverter surge is a phenomenon in which reflection occurs at a breakpoint of impedance, for example, at a starting end, a termination end, or the like of a connected wire in a propagation system, and as a result, a voltage up to twice as high as an inverter output voltage is applied. In particular, an output pulse occurred due to a high-speed switching device, such as an IGBT (Insulated Gate Bipolar Transistor), is high in steep voltage rise. Accordingly, even if a connection cable is short, the surge voltage is high, and further, voltage decay due to the connection cable is low. As a result, a voltage almost twice as high as the inverter output voltage occurs.
For coils for electric equipment such as inverter-related equipment, for example, high-speed switching devices, inverter motors and transformers, insulated wires, which are enameled wires, are mainly used as magnet wires. Accordingly, as described above, since a voltage nearly twice as high as the inverter output voltage is applied in the inverter-related equipment, it has been required in the insulated wires to minimize partial discharge deterioration, which is attributable to inverter surge.
In general, the term “partial discharge deterioration” means a phenomenon in which the following deteriorations of an electric insulating material occur in a complicated manner: molecular chain breakage deterioration caused by collision with charged particles that have been generated by partial discharge (discharge at a portion in which fine void defect and the like exist); sputtering deterioration; thermal fusion or thermal decomposition deterioration caused by local temperature rise; or chemical deterioration caused by ozone generated due to discharge, and the like. The electric insulating materials which actually have been deteriorated by partial discharge show reduction in the thickness.
On the other hand, with downsizing and becoming higher frequency of the electric equipment, in recent years, rectangular wires which are able to increase a space factor have attracted attention. For example, the application of the rectangular wire to motors for vehicles has been advanced. In the rectangular wires, when these wires lie adjacent to each other, air gaps (voids) are easily formed at a corner portion, and further, when a curvature radius of a conductor corner is small, electric field concentration is caused, so that a partial discharge easily generates.
In order to solve a problem of the above partial discharge, an attempt has been made to increase the thickness of an insulating film of the rectangular wire. For example, to coat an enameled rectangular wire with a thermoplastic resin is proposed (see Patent Literature 1). However, to increase the thickness of the insulating film decreases a space factor of the wire and thus has room for improvement. Further, an attempt has been made to decrease a dielectric constant of the insulating film (see Patent Literatures 2 and 3). However, even the case where these resins are used in the insulating film still has room for improvement in a partial discharge inception voltage.